The detection of some voice-activity appears to be a key problem for telecommunication engineers since more and more equipments, ranging from the simpler fax machines to the more sophisticated voice-procession application systems, are likely to be connected to a common Private Switched Telephone Network (PSTN).
U.S. Pat. No. 5,070,526 is an example of a prior art system for analysing a digital representation of a signal to detect call progress signals which are periodic as compared to voice signals which are of essentially random frequency content within the audible frequency band. In this system, an analog communication signal transmitted over a telephone line is converted to digital form for storage in the memory of a computer. Segments of the digital representation of the signal are analysed to determine whether or not the signal has uniform frequency components repeating at regular intervals.
The problem of voice-activity detection is particularly important in the case of voice-server applications providing the transmission of an understandable vocal messages from one system on one side of the telephone line to a human being on the other side. Two different types of calls can be found in such systems according the caller. A first type is often referred to as an "in-coming" call which applies to the telephone communications which are initiated by a person who has dialed the telephone number of a voice-server application, in the case of mail-order business application for instance. Conversely, in some more sophisticated systems, the call may also be initiated by the machine which performs the required dialing operations in order to call a private telephone subscriber and delivers some vocal messages. The second type of call is referred to as a "out-going" call. Considering, for instance, the case of a air-flight booking service application as illustrated in FIG. 1. A voice server 101 provides some air-flight reservation services is connected to a PSTN network 102 and can therefore contact a determined private telephone set 103. Assuming that one specific flight has been subject of a cancellation, it is highly desired that all the persons having booked one seat been informed of the cancellation that has occured. Therefore, the voice-server 101 will have to initiatie a serie of telephone communication calls by dialing the telephone numbers of the telephone subscribers being registered for the considered cancelled flight, manage the different call-progress procedures, recognize whether a person is on the other side of the line, and, at last, delivers a vocal message informing that person of the cancellation that has occured. In the case where the telecommunication call did not succeed, the voice-server will try later on.
Multiple tones are likely to be used in the telephone systems as there are the various control and call status information which are commonly known as call progress tones which are typically transmitted by a central office and transmitted to a calling party in order to keep this party informed of the current status of the call.
Additionaly, considering the fact that such voice-server applications are likely to be used on an international scale, thoses systems must absolutely be capable of handling the numerous call-progress procedures which exist in the world. Therefore, the system will have to process and recognize the wide number of existing tones which are used in the different PSTN existing in the world. A high level of discriminiation is desired in order to provide an accurate discrimination of the different tones conveyed through the network, i.e. the dial-tone, ring-back tone, busy tone, alerting tones, reorder tone, etc. . . . having a different frequency value in each country.
Considering the common dial-tone signal: while the dial-tone signal has a frequency value of 440 Hz in France, the same consists of a 425 Hz tone having a specific cadence in Italy, and is a continuous tone made by combining signals having frequencies of 350 Hz and 440 Hz in the U.S.
Further, since the call may be also be completed with the use of the Dual Tone Multi-Frequency (DTMF) tones, particularly in use in advanced facsimile systems or still rapidly expanding voice-server applications, being one of sixteen combinations of four frequencies from a high-frequency group and four frequencies from a low-frequency group, which is issued when a push button is depressed on the telephone and familiarly called a push tone or a dial tone.
Generally speaking, the tone determination and voice activity detection in the telecommunication systems are either based on a sophisticated Fast Fourier Transformation (FFT), or the use of digital band-pass filters being adjusted at the desired frequencies to be detected. The former systems necessitate a great deal of digital processing resources and the latter can not easily adjusted to match the wide range of tones which exists in the different countries.
Therefore, there is a need in the telecommunication industry for a simple voice-detection process which additionaly provides the detection of the call-progress tones which are likely to exist in the different countries of the world and does not require a great amount of digital processing resources.